Different mobs may appear depending on the age the player is in. Any mob from a previous age may appear in a current age, though their equipment may vary. When ages are changed, old gear will not disappear, it will simply not appear on new spawning mobs. Equippable mobs may spawn with any item from the current age.
Stone Age (Age 0)[]
Passive mobs: Virtually everything. Cows, sheep, llamas, squids, parrots, bats, etc.
Aggressive mobs: Wolves, bears, trumpet skeletons, creepers, spiders, zombies, shoggoths (only spawn in lairs)
Darkland biome mobs: Shadow creatures, abyssal zombies, and other abyssal creatures
Items: Sharp bones (40% chance) or trumpets (skeleton only, unknown chance)
Armour: Leather (50% chance) or wolf armour (15% chance)
Bronze Age (Age 1)[]
Mobs: Slimes, regular skeletons, and skeleton horses
Items: Stone sword (40% chance)
Armour: Leather (50% chance) or wolf armour (25% chance)
Medieval Age (Age 2)[]
Any dimension mobs: Endermen
Hunting dimension mobs: Enderminy, Fallen Mount/Knight, Frozen Skeleton, Wither Witch/Cat
Betweenlands and Twilight Forest mobs: all
Items: Iron sword (50% chance) or gold sword (25% chance), bows (~100% of skeletons)
Armour: Iron (50% chance) or gold (25% chance)
Industrial Age (Age 3)[]
The hunting dimension provides a large number of hazards that can be quite useful for stocking up on drops and experience. Or also quite effective at getting you killed
Hunting dimension mobs: Fat Cats (yes, this is a thing. Extremely large cats that will kill you.)
Nether mobs: Blazes, pigmen, ghasts, spinouts, heatscar spiders, lava golems, wither skeletons, magma cubes
Items: Iron sword (50% chance) or gold sword (25% chance)
Armour: Iron (50% chance) or gold (25% chance)
Modern Age (Age 4)[]
Mobs: Player skin mobs. Some hunting dimension mobs may now spawn in the overworld, albeit they are more rare.
Items: Iron sword (50% chance) or gold sword (25% chance)
Armour: Iron (50% chance) or gold (25% chance)
Futuristic Age (Age 5)[]
Space mobs: Evolved creepers, skeletons, zombies, witches, and endermen
Items: Diamond sword (50% chance)
Armour: Diamond (50% chance)